Phosphor materials that have been and are being used in various display apparatus are mainly oxides and sulfides. However, such phosphor materials are still accompanied by problems in terms of stability and light emitting characteristics. Particularly, high energy needs to be exploited from ultraviolet rays and electron beams for such materials to emit visible light. GaInN type blue lasers have been developed in recent years and white LEDs have been and are being developed by using such lasers as sources of excitation light.
Garnet that is an oxide and expressed by chemical formula ((Y, Gd)3 (Al, Ga)5O12:Ce), to be referred to as YAG:Ce hereinafter) is known as a material that emits yellow light when irradiated with a blue laser beam. The phosphor is obtained from Y—Al-Garnet by partly substituting Y and Al for Gd and Ga respectively and at the same time doping it with activating ions Ce3+ (Non-Patent Document 1). The maximum intensity of emitted light of the yellow phosphor is obtained at or near 550 nm. The yellow phosphor has a problem that it is only possible to obtain pale light when a blue laser beam emitted from it is mixed with excitation light because white light obtained by the mixing contains a red component only to a light extent and thus, the field that can be used is limited in terms of the color rendering characteristic.
Meanwhile, phosphors formed by using a nitride or an oxynitride as host crystal and a rare earth metal as activating metal are attracting attention worldwide. It has been found that an oxynitride produced by replacing part of an oxide by nitrogen for substitution shows a spectrum shifted toward the long wavelength side for excitation light and emitted light from the corresponding spectrum of the oxide because the covalent-bond-originating properties are enhanced (Non-Patent Document 2).
α-Si3N4 is unstable and turns to the β-type by phase transition at high temperature. However, Li, Ca, Mg, Y or a lanthanide metal interstitially fit into the compound to form a so-called interstitial solid solution. The α-sialon that is produced as a result of such a process is stable at high temperature.
It has been known before this patent application that, when the stabilizing metals interstitially existing in an α-sialon to form a solid solution is partly replaced for substitution by optically active metal ions, the spectrum of the α-sialon is shifted to the long wavelength side to give rise to light emission with a red shift (Non-Patent Documents 3 and 4).
One of the inventors of the present invention (Mitomo) also found that a fluorescent material prepared by using a Ca-α-sialon as base material and doping it with Eu2+ turns to a material that emits yellow light when irradiated with visible light in the purple-blue wavelength range (Patent Documents 1 and 2).
This material emits yellow light when blue light, which shows the complementary color of yellow, is irradiated from a blue LED as excitation light so that it is found that it can be used as phosphor for a white light LED when two lights are mixed with each other (Patent Document 3).
However, the above listed materials still have a problem that the extent to which Eu2+ is interstitially fitted into the lattice of α-sialon is still small and hence the intensity of light emitted from them is not satisfactory. A phosphor obtained by doping an Li-α-sialon with Eu2+ is also disclosed (Patent Document 4), although the intensity of light emitted from it does not get to any practical level. This is presumably because a solid solution reaction takes place at high temperature and hence the added Li and Eu interstitially fit into the lattice only to a small extent.
The α-sialon is a solid solution of α-Si3N4. There are two large spaces having a diameter of about 0.1 nm in each unit interstice of the crystal structure of α-Si3N4. Then, the crystal structure is stabilized as metal atoms interstitially fit into the spaces of α-Si3N4 to form a solid solution. Therefore, the general formula of lithium-containing α-sialon is (Lix, My) (Si12−(m+n)Alm+n) (OnN16−n) and hence the number of metal atoms (x+y) that can be interstitially fitted into each unit space to form a solid solution is two at most.
The value of m in the general formula of α-sialon corresponds to the number of Al—N bonds that is substituted for the Si—N bonds of the α-Si3N4 structure and is expressed by m=x+δy (where δ is the valence number of the metal M). Then, n in the general formula represents the number of Al—O bonds that is substituted for the Si—N bonds. Metal-nitrogen bonds take a major part in the α-sialon and hence the α-sialon is a solid solution showing a high nitrogen content ratio.
The electric neutrality is maintained by the lattice substitution and the metal fit-in type solid solution phenomenon. The wavelength of excitation light and that of emitted light of an α-sialon having a particular composition are shifted (red shift) to the long wavelength side due to the characteristics of the crystal structure and the nitrogen-rich chemical composition thereof.
When Si3N4—AlN—Eu2O3—CaO mixture powder is heated in a nitrogen gas flow at 1,750° C. to 1,850° C., a lower melting point liquid is produced out of it. Then, the solid and the liquid phase react with each other when heated further or as time passes to ultimately produce an Eu2+-doped Ca-α-sialon. The trivalent Eu that is added in the heating process is reduced to become divalent Eu which is optically active. With conventional materials of the above-described type, the reaction does not proceed satisfactory because the quantitative ratio of the transitional liquid phase is small and the melting point is high.
Meanwhile, white LEDs are being used in the field where reliability is essential such as security lights and signal lights, in the field where lightweight and downsizing is required such as car-borne lights and the backlights of mobile phones and in the field where visibility is vital such as guide plates in railway stations.
Such white LEDs are adapted to emit white light by mixing lights of different colors. More specifically, blue light emitted from a light emission source, which is a blue LED, with a wavelength of 380 nm to 480 nm and yellow light emitted from a phosphor are mixed to produce white light. The phosphor that is good for use for a white LED is thinly coated on the surface of a blue LED, that operates as light emission source.